Effects of Graphite Morphology and Matrix Structure on Thermal Cycle Fatigue Properties of Cast Iron

When considering material strengthening, full knowledge of the mechanism of destruction is required to develop the strengthening method. Although thermal fatigue is a problem faced for cast iron engine parts due to pouring operations or the operating environment and many researches on thermal fatigue of cast iron have been reported, little research has been conducted on the effects of graphite morphology and matrix structure on thermal fatigue properties. In this research, the effects of graphite morphology and matrix structure on thermal cycle fatigue properties of cast iron were investigated in thermal cycle fatigue tests on thermal fatigue at the explosive side using a sample resembling a cylinder head made of cast iron. The results showed that cracks generated by the thermal cycle fatigue of cast iron are closely related to graphite shape and generate early in the order of flake shape, CV, and spheroidal shape. The progress of cracks was also found to be strongly related to thermal stress generated by heat load and static strength ratio. The difference in the size of thermal stress generated in cast iron is governed by the physical properties of the material. Since physical properties are related to the graphite morphology, the thermal cycle fatigue properties of cast iron also depend on the graphite form. The effects of heat conduction are especially large, and since CV graphite cast iron showed good heat conduction, and exhibited static strength, the thermal cycle fatigue properties of CV graphite cast iron were found to be excellent in moderate cooling conditions.